For the purpose of improving a fuel efficiency and the like, an engine including a plurality of cylinders conventionally performs a reduced-cylinder operation in which: a part of the cylinders is set to a stop state where opening of an intake valve, opening of an exhaust valve, and combustion in the cylinder are stopped; and the engine is driven only by the remaining cylinders.
To return the operation of the engine from the reduced-cylinder operation to an all-cylinder operation in which all the cylinders are operated (the combustion is performed in each of all the cylinders), when the combustion in the stopped cylinder is restarted in a state where the exhaust valve of the stopped cylinder is not open, so-called backfire may occur at the time of the opening of the intake valve, that is, a high-temperature exhaust gas generated by the combustion may flow backward through an intake passage at the time of the opening of the intake valve. Therefore, it is preferable that whether or not the exhaust valve is actually open be determined when the operation of the engine is returned to the all-cylinder operation.
As a device that determines whether or not the exhaust valve is actually open, for example, PTL 1 discloses a device configured to: detect an increased amount of intake pressure increased when the intake valve is open; when the increased amount is large, determine that a close failure of the exhaust valve occurs, and the exhaust valve is not actually open. To be specific, when the close failure of the exhaust valve occurs, a high-pressure combustion gas is not discharged but remains in the cylinder. When the intake valve is open, the high-pressure gas flows backward to the intake passage, and the intake pressure increases. Therefore, when the increased amount is not less than a predetermined value, it is determined that the close failure of the exhaust valve occurs.